On a Win7 Pro system I'm trying to create a BIBM 1.42 USB boot disk using an old but known good thumb drive (formatted FAT32; I can copy files to/from). But when I attempt to create the disk, Makedisk refuses to do so, saying, "Drive M is in use and cannot be accessed."

No, drive is not in use (other than being plugged into the system). I have tried three different USB ports, both USB 2 and USB 3, and the result is the same each time after fresh Windows logoff and logon.

On a Win7 Pro system I'm trying to create a BIBM 1.42 USB boot disk using an
old but known good thumb drive (formatted FAT32; I can copy files to/from).
But when I attempt to create the disk, Makedisk refuses to do so, saying,
"Drive M is in use and cannot be accessed."

No, drive is not in use (other than being plugged into the system). I have
tried three different USB ports, both USB 2 and USB 3, and the result is the
same each time after fresh Windows logoff and logon.